I'm very intrigued by Jack Elliano's 300B design. It's one of those "five parts in a box" kinds of amplifiers. I find the simplicity utterly cool. I do, however, not like that I need 5~6 expensive, heavy, bulky magnetic components to build it, though. I also don't like the use of a 50 W rheostat/variable resistor.

So I was thinking... How about adjusting the bias by adjusting the cathode voltage of the input tube. I came up with attached. I'm curious what you think of it. My main concern is that tube-to-tube variation and drift/aging of the input tube will make the bias of the 300B difficult to adjust and cause it to drift rather quickly. I prefer a "set and forget" kind of design. How valid is this concern?

You don't need the B+ on the first stage, simply ground the top of the CCS and use a negative supply below the Cathode LED's. Depending on the driver conditions, a small resistance may be needed in series with the CCS to get the proper bias for the 300B.

With this setup the CCS / resistor setup will assure that the bias voltage remains constant so the current in the output tube will be determined by combination of B+ and individual tube characteristics.

Yeah... I have noticed that the THD goes up beyond tolerable when the tube draws grid current. It's not a desirable operating point for sure. I'm happy as there's no farting out (blocking distortion).

Do you have any concerns with letting the CCS/input tube combo set the bias for the 300B?

~Tom

I think the idea has considerable merit, my only concern is with what happens as the driver tube ages and the transconductance falls with the potential drop in effective bias at the grid of the 300B. The only thing I would probably do is meter the "cathode" current in the 300B which is a nice bit of additional bling on the chassis. (Can't hurt, and there are tons of inexpensive Chinese made meters for this purpose on eBay.) The other possibility would be to use a CCS on the plate supply to the output stage with a nice big cap to ground to define the source impedance, while the CCS sets the DC operating point and plate current can never run away regardless of what the driver tube is doing. (I'm experimenting with the same idea in the driver stage of my GM70 - no idea how it sounds yet, but it works great on paper.)

This was done originally by Oldeurope (former member) for a 300b. Once you dial in the parts values, it's very stable - essentially self settling. Two tubes and a few resistors and caps (which can all be film - even the power supply).

In the sim, I end up with roughly 420 V across the 300B and an idle current of 100 mA. The 5842 tube runs at 11 mA.

~Tom

Hi Tom,
The dcr of that choke is going to have to be at least 7 - 8K to develop the required bias and I would worry (as I do) about the tempco of that resistance. (Although it will drift in the safe direction.) The other concern is the 5842 itself, operating points all over the map from sample to sample with fixed bias due to the very high slope of the transconductance I do use the 5842 choke loaded with 1.8V led bias on a 100V supply to drive 71A in my headphone amp, but I selected tubes that met the design criteria, and its AC coupled..

One other thing is that pd is rated at 40W for a generic 300B and other than the JJ I have not had good luck with affordable types running at over 70mA.. I run my JJ at 400V/80mA and they have run about 5000hrs so far with no sign of failure. Some types will not run safely at 400V at any current. (Older Shuguang, Valve Arts, etc.)

The other possibility would be to use a CCS on the plate supply to the output stage with a nice big cap to ground to define the source impedance, while the CCS sets the DC operating point and plate current can never run away regardless of what the driver tube is doing.

One suggestion: Try running the cap to the B+ supply rather than to ground. It'll keep the top end of the OPT primary referred to B+ rather that GND. I can't decide if that's an improvement or not, but it's definitely something I'd try. It may improve the supply sensitivity.